This invention relates generally to detecting a boil dry condition in a utensil being heated on a cooking appliance and more particularly to boil dry detection in glass-ceramic cooking appliances.
The use of glass-ceramic plates as the cooking surface in cooking appliances such as cooktops and ranges is well known. Such cooking appliances (referred to herein as glass-ceramic cooktop appliances) typically include a number of heating elements or energy sources mounted under the glass-ceramic plate and an electronic controller. The glass-ceramic plate presents a pleasing appearance and is easily cleaned in that its smooth, continuous surface lacks seams or recesses in which debris can accumulate. The glass-ceramic plate also prevents spillovers from falling onto the energy sources below. When a user selects a power setting for one of the energy sources, the controller ordinarily will cause an appropriate level of power to be supplied to the energy source. This is referred to as the open-loop mode of operation.
In one known type of glass-ceramic cooktop appliance, the glass-ceramic plate is heated by radiation from one or more of the energy sources disposed beneath the plate. The glass-ceramic plate is sufficiently heated by the energy source to heat utensils placed on it primarily by conduction from the heated glass-ceramic plate to the utensil. Another type of glass-ceramic cooktop appliance uses an energy source that radiates substantially in the infrared region in combination with a glass-ceramic plate that is substantially transparent to such radiation. In these appliances, a utensil placed on the cooking surface is heated partially by radiation transmitted directly from the energy source to the utensil, rather than by conduction from the glass-ceramic plate. Such radiant glass-ceramic cooktop appliances are more thermally efficient than other glass-ceramic cooktop appliances and have the further advantage of responding more quickly to changes in the power level applied to the energy source. Yet another type of glass-ceramic cooktop appliance inductively heats utensils placed on the cooking surface. In this case, the energy source is an RF generator that emits RF energy when activated. The utensil, which comprises an appropriate material, absorbs the RF energy and is thus heated.
In each type of glass-ceramic cooktop appliances, provision must be made to avoid overheating the glass-ceramic plate. For most glass-ceramic materials, the operating temperature should not exceed 600-700xc2x0 C. for any prolonged period. Under normal operating conditions, the temperature of the glass-ceramic plate will generally remain below this limit. However, conditions can occur during open-loop mode operation that can cause this temperature limit to be exceeded. Commonly occurring examples include operating the appliance with a small load or no load (i.e., no utensil) on the cooking surface, using badly warped utensils that make uneven contact with the cooking surface, and operating the appliance with a shiny and/or empty utensil.
To protect the glass-ceramic plate from extreme temperatures, a control system is utilized in which temperature sensors provide a signal indicative of the glass-ceramic temperature to the appliance""s controller. If the glass-ceramic plate approaches its maximum temperature, the controller switches from the open-loop mode to a special control algorithm, known as the thermal limiter mode. In the thermal limiter mode, the controller overrides the user power settings and reduces power to the energy sources to maintain the temperature of the glass-ceramic cooking surface at a relatively constant, safe temperature.
Another concern with cooking appliances generally is a boil dry condition. A boil dry condition occurs when all the liquid contents of a heated utensil evaporate during the boil phase. This commonly happens when a utensil is inadvertently left on a hot cooking surface or otherwise overheated. A boil dry condition can cause burned food, utensil damage and potential fire hazards. Accordingly, automatic detection of a boil dry condition is a desirable feature in cooking appliances.
In glass-ceramic cooktop appliances, it is known to use the glass ceramic temperature to determine when a utensil has boiled dry. Specifically, when a utensil containing water or another liquid is placed on a glass-ceramic cooking surface and the burner is turned on, the glass-ceramic temperature initially increases rapidly. The glass-ceramic temperature will continue to rise until the utensil contents come to a boil. During the boil phase, the utensil contents will boil off at a steady temperature and remove excess heat via evaporation. With this steady heat removal, the glass-ceramic temperature also reaches a steady state value some time after the contents come to a boil. However, when the liquid completely boils off, there is a sudden drop in heat removal from the pan, and consequently, the glass-ceramic temperature increases rapidly. This temperature spike is thus indicative of the boil dry condition.
One known approach to detecting the sudden rise in temperature is to monitor the first derivative of the output of the glass-ceramic temperature sensor. However, this signal is quite noisy and evaluating the first derivative of the temperature signal with respect to time can produce a highly corrupted estimate. Furthermore, monitoring glass-ceramic temperature will not effectively detect boil dry events in the thermal limiter mode. This is because the controller maintains the glass-ceramic plate at a relatively constant temperature during thermal limiter operation. Therefore, the glass-ceramic temperature will not spike when a boil dry condition occurs.
Accordingly, it would be desirable to be able to accurately detect boil dry events under both the open-loop and thermal limiter modes.
The above-mentioned need is met by the present invention, which provides a boil dry detection system for a cooking appliance having at least one energy source disposed under a cooking surface and a power source for providing power to the energy source. The boil dry detection system includes a means for providing a signal representative of a parameter associated with the cooking appliance and a means for generating an estimate of the first and second derivatives of the signal with respect to time. The system further includes a means for producing the cross-correlation of the first and second derivative estimates and a means for providing a boil dry indication when the cross-correlation exceeds a predetermined threshold.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.